1. Field of the Invention
The present invention relates to magnetic amorphous alloys and to a method for annealing these alloys in a magnetic field simultaneously applying a tensile stress. The present invention is also directed to making amorphous magnetostrictive alloys for use in a marker in a magnetomechanical electronic article surveillance or identification.
2. Description of the Prior Art
U.S. Pat. No. 5,820,040 teaches that transverse field annealing of amorphous iron based metals yields a large change of Young's modulus with an applied magnetic field and that this effect provides a useful means to achieve control of the vibrational frequency of an electromechanical resonator with the help of an applied magnetic field.
The possibility to control the vibrational frequency by an applied magnetic field described in European Application 0 093 281 as being particularly useful for markers for use in electronic article surveillance. The magnetic field for this purpose is produced by a magnetized ferromagnetic strip (bias magnet) disposed adjacent to the magnetoelastic resonator, with the strip and the resonator being contained in a marker or tag housing. The change in effective permeability of the marker at the resonant frequency provides the marker with signal identity. This signal identity can be removed by changing the resonant frequency by changing the applied field. Thus, the marker, for example, can be activated by magnetizing the bias strip and, correspondingly, can be deactivated by degaussing the bias magnet which removes the applied magnetic field and thus changes the resonant frequency appreciably. Such systems originally (cf. European Application 0 0923 281 and PCT Application WO 90/03652) used markers made of amorphous ribbons in the as prepared state which also can exhibit an appreciable change of Young's modulus with an applied magnetic field owing to uniaxial anisotropies associated with production-inherent mechanical stresses.
U.S. Pat. No. 5,469,140 discloses that the application of transverse field annealed amorphous magnetomechanical elements in electronic article surveillance systems removes a number of deficiencies associated with the markers of the prior art which use "as prepared" amorphous material. One reason is that the linear hysteresis loop associated with the transverse field annealing avoids the generation of harmonics which can produce undesirable alarms in other types of EAS systems (i.e. harmonic systems). Another advantage of such annealed resonators is their higher resonant amplitude. A further advantage is that the heat treatment in a magnetic field significantly improves the consistency in terms of the resonant frequency of the magnetostrictive strips.
As, for example, explained by Livingston J. D. 1982, "Magnetomechanical Properties of Amorphous Metals", phys. stat. sol. (a) vol 70, pp 591-596 or by Herzer, G. (1997), Magnetomechanical damping in amorphous ribbons with uniaxial anisotropy, Materials Science and Engineering A226-228, pp. 631, the resonator properties, such as resonant frequency, the amplitude or the ring-down time are largely determined by the saturation magnetostriction and the strength of the induced anisotropy. Both quantities strongly depend on the alloy composition. The induced anisotropy additionally depends on the annealing conditions i.e. on annealing time and temperature and a tensile stress applied during annealing (cf. Fujimori H., 1983 "Magnetic Anisotropy" in F. E. Luborsky (ed) Amorphous Metallic Alloys, Butterworths, London, pp. 300-316 and references therein, Nielsen O., 1985, Effects of Longitudinal and Torsional Stress Annealing on the Magnetic Anisotropy in Amorphous Ribbon Materials, IEEE Transactions on Magnetics, vol Mag-21, No 5, Hilzinger H. R., 1981, Stress Induced Anisotropy in a Non-Magnetostrictive Amorphous Alloy, Proc. 4th Int. Conf. On Rapidly Quenched Metals (Sendai 1981), pp. 791). Consequently, the resonator properties depend strongly on these parameters.
Accordingly, aforementioned U.S. Pat. No. 5,469,140 teaches that a preferred material is an Fe--Co-based alloy with at least about 30 at % Co. The high Co-content according to this patent is necessary to maintain a relatively long ring-down period of the signal. In German Gebrauchsmuster G 94 12 456.6 it was recognized that a long ring down time is achieved by choosing an alloy composition which reveals a relatively high induced magnetic anisotropy and, that, therefore, such alloys are particularly suited for EAS markers. This Gebrauchsmuster teaches that this can also be achieved at lower Co-contents if, starting from a Fe--Co-based alloy, up to about 50% of the iron and/or cobalt is substituted by nickel. U.S. Pat. No. 5,728,237 discloses further compositions with Co-content lower than 23 at % which are characterized by a small change of the resonant frequency and the resulting signal amplitude due to changes in the orientation of the marker in the earth's magnetic field and which at the same time are reliably deactivatable. The need for a linear loop with relatively high anisotropy and the benefit of alloying Ni in order to reduce the Co-content for such magnetoelastic markers was reconfirmed by the disclosure of U.S. Pat. No. 5,628,840 which teaches that alloys with an iron content of at least 30 at % and below about 45 at % are particularly suited.
The field annealing in the aforementioned examples was done across the ribbon width i.e. the magnetic field direction was oriented perpendicularly to the ribbon axis and in the plane of the ribbon surface. This technique will be referred to as transverse field-annealing. The strength of the magnetic field has to be strong enough in order to saturate the ribbon ferromagnetically across the ribbon width. This can be already achieved in magnetic fields of a few hundred Oe. U.S. Pat. No. 5,469,140, for example, teaches a field strength in excess of 500 Oe or 800 Oe, respectively; similarly PCT Application WO 96/32518 discloses a field strength of about 1 kOe to 1.5 kOe. Such transverse field-annealing can be performed, for example, batch-wise either on toroidally wound cores or on pre-cut straight ribbon strips. Alternatively and as disclosed in detail in European Patent Application 0 737 986 corresponding to (U.S. Pat. No. 5,676,767), the annealing can be advantageously performed in a continuous mode by transporting the alloy ribbon from one reel to another reel through an oven in which a transverse saturating field is applied to the ribbon.
Typical annealing conditions disclosed in aforementioned patents are annealing temperatures from about 300.degree. C. to 400.degree. C.; annealing times from several seconds up to several hours. PCT Application WO 97/13258, for example, teaches annealing speeds from about 0.3 m/min up to 12 m/min for a 1.8 m long furnace.
Aforementioned PCT Application WO 96/32518 also discloses that a tensile stress ranging from about zero to about 70 MPa can be applied during annealing. The result of this tensile stress is that the resonator amplitude and the frequency slope .vertline.df.sub.r /dH.vertline. either slightly increases, remains unchanged or slightly decreases, i.e., there was no obvious advantage or disadvantage for the resonator properties when applying a tensile stress limited to a maximum of about 70 MPa.
It is well known (cf. the aforementioned Nielsen article and Hilzinger article) that a tensile stress applied during annealing induces a magnetic anisotropy. The magnitude of this anisotropy is proportional to the magnitude of the applied stress and depends on the annealing temperature, the annealing time and the alloy composition. The anisotropy orientation corresponds either to a magnetic easy ribbon axis or a magnetic hard ribbon axis (the easy magnetic plane being perpendicular to the ribbon axis) and thus either decreases or increases the field induced anisotropy depending on the alloy composition.
The fingerprint of the aforementioned markers, as well as for other magneto-acoustic markers used e.g. in identification systems is their resonant frequency at a given bias field.
One problem is that the resonant frequency can be subject to changes due to the orientation of the marker in the earth's magnetic field and/or due to scatter in the bias magnet's properties. Thus, for aforementioned EAS markers, it is highly desirable that the resonant frequency f.sub.r in the activated state (i.e. when the bias magnet is magnetized) varies as little as possible with the applied magnetic field H--a typical requirement e.g. is .vertline.df.sub.r /dH.vertline.&lt;700 Hz/Oe. This requires a relatively high magnetically induced anisotropy which can be only achieved when the resonator alloy contains an appreciable amount of Co and/or is annealed at relatively low annealing speeds. However, because of the high raw material cost of cobalt, it is highly desirable to reduce its content in the alloy. High annealing speeds are a further requirement to reduce production and investment cost.
Another problem is that the resonant frequency at a given bias and the change of the resonant frequency with the bias field are highly sensitive to a variety of parameters. Apart form the length and the width of the resonator, these parameters include the chemical composition, the thickness of the resonator and the time and temperature of the heat treatment. Thus, in order to guarantee reproducible resonator properties from batch to batch a composition must be reproduced with an accuracy beyond the capability of chemical analysis. Similarly, in order to guarantee reproducible resonator properties within one batch thickness fluctuations must be restricted to less than .+-.1 .mu.m, which is at the limit or even beyond the limit of current manufacturing technology. Finally, reproducible properties require a most precise control of the annealing temperature and annealing time which both sensitively influence the resonator properties. Clearly these circumstances require most narrow tolerances in the whole manufacturing line, limit the production yield and, thus, enhance manufacturing cost significantly.